Modern Americans are more sentimental than they were 100 years ago, so we're unwilling to shoot them all. Were I a horse I'd rather be shot than starve or be eaten alive by wolves, but nobody asks the horses.

So why can't we hire cowboys to shoot mares with Depo-Provera in late May? It's cheap stuff, its used with horses, and it's designed for deposition. Shoot a capsule of it into the mare buttocks around mating time.